The present invention relates to an impedance varying unit which is built in a microwave cavity entrapping microwave or built in a waveguide which propagates microwave. The unit works on the microwave distributed throughout the structure of the cavity or the waveguide to change the propagation details of the wave. The present invention also relates to a microwave device as well as a high frequency heating appliance both using this impedance varying unit.
One of the methods to change the propagation details of the microwave traveling through a conventional waveguide is to place a metal member in a traveling path of the microwave in the waveguide, so that capacitive component or inductive component as impedance is added in the waveguide. For instance, the metal member is inserted from H plane (wide wall) in roughly parallel to E plane (narrow wall) to produce capacitive component within the waveguide, so that transmission characteristics of the microwave travelling the waveguide are changed.
In this waveguide, the metal member is movable with respect to metal walls constituting the waveguide. Some creative ideas and a given clearance are provided between the movable metal member and the walls of the waveguide in order to eliminate spark-occurrence therebetween. Regarding the clearance, an electric-wave-shielding-mechanism is provided in order to prevent the leakage of an electric wave from the clearance. Accordingly, the conventional waveguide requires a complicated and a large size construction due to this movable metal member.
There is another method for changing the transmission details of the microwave in the microwave cavity. For instance, the location of supplying section of the microwave to the microwave cavity is changed, or a method of coupling (magnetic field coupling or electric field coupling) with the microwave cavity is changed, so that standing-wave-distribution produced in the microwave cavity is changed.
In the microwave cavity explained above, when a distribution of the standing wave is changed, it is necessary to change physically the location of the supplying section or the coupling method. As a result, it is difficult to change the distribution of standing wave in time sequence.
Further, in a high frequency heating appliance, so called xe2x80x9cmode stirrerxe2x80x9d is available, which spins metal blades to reflect diffusely microwave in the microwave cavity where an object to be heated is placed. The mode stirrer reflects the microwave incident upon the blades in every direction, and it is difficult to reflect the wave into a specific area. Therefore, it is difficult to selectively provide inducible heat to a specific area of the object.
Still further, in a high frequency heating appliance, xe2x80x9cmode turntablexe2x80x9d is available, which rotates the object to be heated on a table in the microwave cavity. The mode turntable is to promote uniform heating by moving the object per se to the standing wave distribution produced in the microwave cavity, and a degree of the heating uniformity depends on the distribution of standing wave. This xe2x80x9cmode turntablexe2x80x9d does not positively change the distribution of the standing wave.
Japanese Patent Laid-Open Publication No. H08-330066 discloses a high frequency heating appliance, in which the direction of high frequency current running on the wall of the microwave cavity is varied, so that an exciting mode produced in the microwave cavity can be changed. This technique uses a plate on which a plurality of openings are arranged, and the plate is disposed on roughly the same plane as the microwave cavity wall, and the plate is driven to spin. Spinning the plate changes the direction of a longitudinal axis, thereby changing the direction of the high-frequency-current running on the microwave-cavity-wall, which also changes an exciting mode in the cavity. This intends to select an exciting mode adequate to the object to be heated, and to result in a heating distribution fit for the object.
However, this technique is formed by rotating an opening, and it is necessary to prevent the production of spark entailed by the spin of the metal member in the microwave cavity. This requires a spark preventive mechanism and makes the construction complicated. A longitudinal length of the opening must be more than a half of the wavelength of the microwave radiated in the cavity, so that a space necessary for rotating the opening occupies a large area on the wall where the opening is disposed. In the construction discussed above, exciting modes available in the cavity are limited due to the influence of the opening or the clearance for preventing the spark. This makes it difficult to obtain a heating distribution fit for the object.
The present invention addresses the problems discussed above, and aims to deal with the problem of spark occurrence and provide a compact size and controllable impedance-varying-unit. The impedance varying unit is mounted on a wall defining a microwave cavity, and the unit controls as well as varies the impedance of the wall, so that standing wave distribution produced in the microwave cavity can be varied in time-sequence. This is done by a microwave device of the present invention. The present invention also provides a high frequency heating appliance using this microwave device, and the appliance can selectively heat an object or realize uniform heating.
The impedance varying unit of the present invention comprises the following elements:
a waveguide section of which first end is terminated and a second end is opened; and
a movable body disposed within the waveguide section.
The movable body is driven and controlled, so that impedance at an open end of the waveguide section can be varied. For that purpose, the operating face of the unit is coupled to an arbitrary boundary of the cavity, so that leakage of electric wave from the operating face is eliminated.
A transmission mode TEn0 is used in the waveguide section (xe2x80x9cnxe2x80x9d is a positive integer), so that the impedance varying unit of the present invention can uniform the effect of incident microwave to the movable body and make the effect independent of xe2x80x9cnxe2x80x9d. As a result, design change such as enlarging the operating face can be done with ease.
Further, the movable body rotates on a rotary shaft defining the space to the terminal end of the waveguide section, so that the movable space of the movable body can be limited to a smaller space. As a result, the unit in a compact size can vary and control the impedance at the open end.
Still further, the movable body can be moved by changing the space to the terminal end of the waveguide section, so that a variable range of the impedance at the open end can be widened.
Yet further, the movable body is made of non-metallic material, so that a breakdown voltage between the body and the waveguide section can be raised. As a result, reliability of the unit against an input of high powered microwave can be guaranteed.
Further, when the impedance at the open end is zero (0), the open end works in the same manner as a metal wall, so that characteristic comparison with a case where the impedance varying unit is not used can be checked with ease.
At the open end, a phase difference between an incident wave to the waveguide section and a reflective wave therefrom becomes zero (0) so that effect of the microwave at the open end can be maximized.
Further, the impedance at the open end forms impedance of inductive component and capacitive component with zero centering in between. Thus the unit can be utilized for matching adjustment.
Still further, in the impedance varying unit of the present invention, a position detector detects a position of the movable body, so that the effect of the microwave at the open end can be positively controlled based on a signal of the position detector. Based on a signal from a microwave detector, the position of the movable body is determined, so that the microwave detector also works as a position detector for the movable body. As a result, other position detectors can be eliminated.
The impedance varying unit of the present invention detects the standing wave distribution in the waveguide section, so that an operation status of the unit can be determined and the reliability of the operation can be guaranteed.
The impedance varying unit of the present invention includes a stepping motor which drives the movable body, so that a stay time at each step can be arbitrarily controlled. As a result, various combinations of impedance including time factor can be controlled.
The impedance varying unit in a flat shape can be integrated into the microwave device even when a length of the waveguide section is long because the terminal end and the open end form approx. right angles.
A plurality of the movable bodies are disposed in the waveguide section and driven, and journal (rotatably supporting) angles of the bodies are variously combined, so that a variable range of the impedance at the open end can be largely varied. As a result, the effect of the microwave can be maximized.
The microwave device of the present invention comprises the following elements:
a microwave cavity practically entrapping microwave fed;
an opening disposed on metal wall forming the cavity; and
the impedance varying unit.
In the cavity, various distributions of standing wave are formed responsive to impedance at the opening.
Further, the microwave device of the present invention comprises the following elements:
a microwave cavity in a shape of approx. rectangular hexahedron practically entrapping microwave fed;
an opening provided on at least one plane of the rectangular; and
the impedance varying unit for varying the impedance at the opening.
The impedance at the opening on the wall of the cavity is changed, so that node and antinode of the standing wave distribution produced in the cavity move linearly.
The microwave device of the present invention can produce multiplexed transmission of microwave or various distributions of standing wave within the cavity by providing the opening as well as the impedance varying unit coupled to the opening.
In the microwave cavity having a plurality of exciting modes, a plurality of openings are disposed so that high-frequency-currents flowing on a wall of the cavity independently and separately flows. The impedance of each opening thus varies, this mechanism allows a desirable single exciting mode to be produced in the microwave cavity.
A high frequency heating appliance comprises the following elements:
the microwave cavity accommodating an object to be heated;
a high frequency generator for radiating microwave into the cavity;
a controller for controlling the impedance varying unit included in the cavity based on heating information of the object, and the high frequency generator.
This construction allows the impedance at the opening or the energy of the microwave generated by the high frequency generator to be controlled, so that a specific area of the object can be selectively heated or the entire object can be uniformly heated.
Further, the high frequency heating appliance of the present invention has a turntable which rotates the object to be heated in the microwave cavity. Controlling the impedance varying unit as well as a table driver controls the distribution of standing wave and the rotation of the object in the cavity. Thus the entire object is heated further uniformly.
The high frequency heating appliance of the present invention controls the impedance varying unit, so that the standing-wave-distribution in the cavity can be varied in time-sequence. This mechanism allows an object difficult for being rotated to be heated uniformly.
The high frequency heating appliance of the present invention automatically controls to provide an optimum heating distribution to the object, so that convenience of the appliance is enhanced.
Further, the high frequency heating appliance of the present invention can increase a varying speed of the impedance varied by the impedance varying unit. As a result, diffusive reflection of microwave in the cavity is promoted, which promotes uniform heating of the object in vertical direction.
In the high frequency heating appliance of the present invention, a user can select and input one of heating methods of the object, namely heating methods including xe2x80x9cdefrostxe2x80x9d, xe2x80x9cre-heatingxe2x80x9d, xe2x80x9coven-heatingxe2x80x9d, and xe2x80x9ckeep warm.xe2x80x9d There are several types of information difficult to be detected by the high frequency heating appliance: they are types of the objects, shapes of the objects, a number of the objects, and a method of heating, i.e. one of defrost, re-heating, oven-heating, and keeping warm. Since the method of heating can be selected and input by a user, the controller recognizes the object roughly dividing into two types, one is a specific area of the object is to be selectively heated, the other is the entire object is to be uniformly heated.
To be more specific, regarding the types of the objects, moisture content of the object is one of references. For instance, the controller recognizes the object having more than 60% of moisture content as requiring overall heating, and controls the impedance varying unit accordingly. Regarding the shapes of the objects, the controller recognizes a flat object as requiring overall heating. Regarding the number of the objects, the controller recognizes a plurality of objects as requiring overall heating. When the xe2x80x9cdefrostxe2x80x9d is input, the controller recognizes the object as requiring overall heating advantageously over other input information. This mechanism realizes the heating desired by a user.
In the high frequency heating appliance of the present invention, the heating information can be obtained as physical information from the high frequency heating appliance or the object to be heated during the heating.
In order to obtain this physical information, the appliance includes detectors corresponding to the physical information. The physical information obtained from the appliance is an intensity signal of electromagnetic field within the microwave cavity or the impedance varying unit. On the other hand, the physical information obtained from the object is at least one of, e.g. weight, present temperature of the object, and density of gas produced from the object.
The physical information discussed above is used for determining details of controlling to be carried out by the controller during the heating.
In other words, intensity information of electromagnetic field is used as heating-control-information for the object difficult to detect its temperature, or for avoiding idle-heating when no object exists. The weight information of the object is used for controlling the heating or determining an end of the heating by detecting a degree of dryness of the object. Temperature information is used for variable control of the impedance at the opening based on the present temperature of the object, or for variable control of microwave power supplied from the high frequency generator, or for detecting an end of heating the object. The density information of gas produced from the object is used for, e.g., in the case of vapor, when the vapor amount reaches a given level, the heating should be finished.
The various information discussed above is used, so that at least one of the impedance varying unit, the table driver and the high frequency generator can be automatically controlled by a signal from the controller, or manually controlled with a manually input signal. Thus the object can be heated selectively or entirely as a user desires.
As explained above, the present invention solves the problem of the spark occurrence, and also provides the highly controllable impedance-varying-unit in a compact body, the microwave device using the unit and the high-frequency-heating-appliance using the unit. When these unit, device and appliance are used, the distribution of standing-wave produced in the microwave cavity can be varied in time-sequence, and the object can be selectively or uniformly heated.